System and Method for the Electrolysis of Carbon Dioxide

ABSTRACT

Various embodiments include a system for carbon dioxide electrolysis comprising: an electrolysis cell having an anode and a cathode comprising a gas diffusion electrode adjoined by a gas space and a cathode space; an electrolyte circuit adjoining the electrolysis cell; a gas supply for supplying carbon dioxide-containing gas to the gas space, the gas space including an electrolyte outlet; and a shutoff device for the electrolyte outlet, wherein the shutoff device opens when a pressure differential between the gas space and the cathode space exceeds a threshold value.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage Application of International Application No. PCT/EP2017/061927 filed May 18, 2017, which designates the United States of America, and claims priority to DE Application No. 10 2016 211 819.6 filed Jun. 30, 2016, the contents of which are hereby incorporated by reference in their entirety.

TECHNICAL FIELD

The present disclosure relates to electrolysis. Various embodiments may include a system and/or a method for carbon dioxide electrolysis.

BACKGROUND

The combustion of fossil fuels currently provides about 80% of global energy demand. These combustion processes in 2011 emitted around 34 000 million tonnes of carbon dioxide (CO2) into the atmosphere globally. This release is the simplest way of disposing even of large volumes of CO2 (large brown coal power plants more than 50 000 t per day). Discussion about the adverse effects of the greenhouse gas CO2 on the climate has led to consideration of reutilization of CO2. CO2 is a strongly bonded molecule and can therefore be reduced back to usable products only with difficulty.

In nature, CO2 is converted to carbohydrates by photosynthesis. This complex process can be reproduced on the industrial scale only with great difficulty. One currently technically feasible route is the electrochemical reduction of CO2. The carbon dioxide is converted here with supply of electrical energy to a product of higher energy value, for example CO, CH4, C2H4 or C1-C4 alcohols. The electrical energy in turn preferably comes from renewable energy sources such as wind power or photovoltaics.

For electrolysis of CO2, in general, metals are used as catalysts. The type of metal affects the products of the electrolysis. For example, CO2 is reduced virtually exclusively to CO over Ag, Au, Zn and, to a limited degree, over Pd and Ga, whereas a multitude of hydrocarbons are observed as reduction products over copper. As well as pure metals, metal alloys are also of interest, as are mixtures of metal and metal oxide having cocatalytic activity, since these can increase selectivity for a particular hydrocarbon.

In CO2 electrolysis, a gas diffusion electrode (GDE) can be used as cathode in a similar manner to that in chlor-alkali electrolysis in order to establish a three-phase boundary between the liquid electrolyte, the gaseous CO2 and the solid silver particles. This is done using an electrolysis cell as also known from fuel cell technology, having two electrolyte chambers, wherein the electrolyte chambers are separated by an ion exchange membrane. The working electrode is a porous gas diffusion electrode. It comprises a metal mesh, to which a mixture of PTFE, activated carbon, a catalyst and further components has been applied. It comprises a pore system into which the reactants penetrate and react at the three-phase interfaces.

The counterelectrode is sheet metal coated with platinum or a mixed iridium oxide. The GDE is in contact with the electrolyte on one side. On the other side it is supplied with CO2 which is forced through the GDE by positive pressure (called convective mode of operation). The GDE here may contain various metals and metal compounds that have a catalytic effect on the process. The mode of function of a GDE is known, for example, from EP 297377 A2, EP 2444526 A2 and EP 2410079 A2.

By contrast with chlor-alkali electrolysis and with fuel cell technology, the product formed in carbon dioxide electrolysis is gaseous and not liquid. In addition, the CO2 used forms salts with the alkali metal or alkaline earth metal hydroxide formed from the electrolyte. For example, when potassium salts are used as electrolytes, KOH is formed, and the salts KHCO3 and K2CO3 are formed. Owing to the operating conditions, there is crystallization of the salts in and on the GDE from the gas side.

The electrochemical conversion of CO2 over silver electrodes proceeds according to the following equation:

Cathode: CO2+2e−+H2O→CO+2OH−

with the counter-reaction

Anode: 6H2O→O2+4e−+4H3O+

Owing to the electrochemical conditions, the charge in the chemical equations is not balanced uniformly with H3O+ or OH−. In spite of acidic electrolyte, locally basic pH values occur at the GDE. For operation of alkaline fuel cell technology, the oxygen introduced has to be CO2-free since KHCO/K2CO3 would otherwise form according to the following equations:

CO2+KOH→KHCO3

CO2+2KOH→K2CO3+H2O

The same process is also observed in CO2 electrolysis, with the difference that the gas fed in cannot be CO2-free. As a result, after a finite time (depending on the current density), salt crystallizes in and on the GDE from the gas side and blocks the pores of the GDE. The gas pressure rises, the GDE is highly stressed and it tears over and above a particular pressure. Moreover, the potassium ions needed for the process are withdrawn from the process and the gas space is gradually filled with salt. An analogous process is observed with other alkali metal/alkaline earth metals, for example cesium.

Stable long-term operation of the gas diffusion electrode in the region of more than 1000 h is not possible in CO2 electrolysis since the salt formed blocks the pores of the GDE and these thus become gas-impermeable.

SUMMARY

The teachings of the present disclosure may enable an improved system for carbon dioxide electrolysis and/or a method of operating an arrangement for carbon dioxide electrolysis, with which stable long-term operation is enabled with avoidance of the disadvantages mentioned at the outset. For example, some embodiments may include an arrangement for carbon dioxide electrolysis, comprising: an electrolysis cell (11) having an anode (13) and a cathode (15), where anode (13) and cathode (15) are connected to a voltage supply (22), where the cathode (15) takes the form of a gas diffusion electrode adjoined on a first side by a gas space (16) and on a second side by a cathode space (14), an electrolyte circuit (20) that adjoins the electrolysis cell (11), a gas supply (17) for supplying carbon dioxide-containing gas to the gas space (16), characterized in that the gas space (16) has an electrolyte outlet (25) and the electrolyte outlet (25) has been provided with a shutoff device (32), configured such that the shutoff device (32) is opened when the pressure differential between gas space (16) and cathode space (14) exceeds a threshold value.

In some embodiments, the shutoff device (32) is a pressure relief valve (32).

In some embodiments, the electrolyte outlet (25) in the gas space (16) is disposed at the bottom end.

In some embodiments, there is a first pressure sensor (31) for the gas space (16).

In some embodiments, there is a second pressure sensor (30) for the cathode space (14).

In some embodiments, there is a pressure differential sensor for gas space (16) and cathode space (14).

In some embodiments, the electrolyte outlet (25) is connected to the electrolyte circuit (20).

In some embodiments, there is a control device (23) configured to control the shutoff device (32) as a function of the pressure differential.

As another example, some embodiments include a method of operating an arrangement for carbon dioxide electrolysis with an electrolysis cell (11) having an anode (13) and a cathode (15), where anode (13) and cathode (15) are connected to a voltage supply (22), where the cathode (15) takes the form of a gas diffusion electrode adjoined on a first side by a gas space (16) and on a second side by a cathode space (14), where carbon dioxide-containing gas is introduced into the gas space (16), characterized in that an electrolyte outlet (25) provided with a shutoff device (32) is provided in the gas space (16), a pressure differential between gas space (16) and cathode space (14) is ascertained, and the shutoff device (32) is opened when the pressure differential exceeds a threshold value.

In some embodiments, the shutoff device (32) is operated such that the pressure differential between gas space (16) and cathode space (14) remains within a definable interval.

BRIEF DESCRIPTION OF THE DRAWINGS

A working example embodiment of the teachings herein, but one which is by no means limiting, is now elucidated in detail with reference to the drawing. The features are shown here in schematic form.

DETAILED DESCRIPTION

In some embodiments, a system for carbon dioxide electrolysis comprises an electrolysis cell having an anode and a cathode, both of which are connected to a voltage supply. The cathode takes the form of a gas diffusion electrode adjoined on a first side by a gas space and on a second side by a cathode space. In addition, the arrangement comprises an electrolyte circuit that adjoins the electrolysis cell and a gas supply for supplying carbon dioxide-containing gas to the gas space. The gas space has an electrolyte outlet and the electrolyte outlet has been provided with a shutoff device. This arrangement is configured such that the shutoff device is opened when the pressure differential between gas space and cathode space exceeds a threshold value.

In some embodiments, a method of the invention for operating an arrangement for carbon dioxide electrolysis with an electrolysis cell having an anode and a cathode, where anode and cathode are connected to a voltage supply, where the cathode takes the form of a gas diffusion electrode adjoined on a first side by a gas space and on a second side by a cathode space, an electrolyte outlet provided with a shutoff device is provided in the gas space, a pressure differential between gas space and cathode space is ascertained and the shutoff device is opened when the pressure differential exceeds a threshold value.

In some embodiments, the voltage-driven electrolyte pumping effect through the gas diffusion electrode permits a solution of simple construction in order to prevent salting-up at the gas diffusion electrode in CO₂ electrolysis. The shutoff device ensures that the pressure differential does not become too high and hence the electrolyte flow through the gas diffusion electrode persists in a lasting manner. As a result, the salts that form are advantageously transported away by the electrolyte in situ. This enables lasting operation of the electrolysis. In some embodiments, the following features can additionally be provided for the arrangement:

In some embodiments, the shutoff device may be a shutoff slide valve, a shutoff flap valve, or ball valve. The shutoff device may be a safety valve (pressure relief valve) or a proportional valve. In some embodiments, a pressure relief valve does not require any control, but opens automatically on exceedance of the threshold value for the pressure differential between gas space and cathode space.

In some embodiments, the electrolyte outlet in the gas space may be disposed at the bottom end, such that outflow of the electrolyte is enabled.

In some embodiments, there is a first pressure sensor in the gas space. This gives a pressure signal, for example, to a control device for actuation of the shut-off device. A second pressure sensor may be disposed within the cathode space. This can likewise give a pressure signal to the control device. The two pressure signals can be used by the control device to determine the pressure differential and undertake control of the shutoff device.

In some embodiments, a pressure differential sensor for gas space and cathode space may be present. This directly gives a signal for the pressure differential to a control device or directly to the shutoff device.

In some embodiments, the electrolyte outlet may be connected to the electrolyte circuit. It is thus possible to subsequently feed the electrolyte discharged via the shutoff device back to the system again. Thus, the electrolyte is also not consumed. The CO₂ feed gas stream is unaffected here, and hence sufficient CO₂ supply to the process is assured.

In some embodiments, there is a control device configured to control the shutoff device as a function of the pressure differential.

In some embodiments, the shutoff device may be operated such that the pressure differential between gas space and cathode space remains within a definable interval. There may be a higher pressure remaining in the gas space than in the cathode space. The interval chosen may be narrow, such that, for example, the pressure differential varies by not more than 10% or not more than 5%.

The construction of an electrolysis cell 11 shown in schematic form in the figure is suitable for undertaking a carbon dioxide electrolysis. This embodiment of the electrolysis cell 11 comprises at least one anode 13 with an adjoining anode space 12, and a cathode 15 and an adjoining cathode space 14. Anode space 12 and cathode space 14 are separated from one another by a membrane 21. According to the electrolyte solution used, a construction without a membrane 21 is also conceivable, in which case pH balancing then goes beyond that by the membrane 21.

Anode 13 and cathode 15 are electrically connected to a voltage supply 22 which is controlled by the control unit 23. The control unit 23 may apply a protection voltage or an operating voltage to the electrodes 13, 15, i.e. the anode 13 and the cathode 15. The anode space 12 of the electrolysis cell 11 shown is equipped with an electrolyte inlet. The anode space 12 depicted likewise comprises an outlet for electrolyte and, for example, oxygen O₂ or another gaseous by-product which is formed in the carbon dioxide electrolysis at the anode 13. In the case of a chloride-containing anolyte, for example, chlorine gas is formed. The cathode space 14 in each case likewise has at least one product and electrolyte outlet. The overall electrolysis product may be composed of a multitude of electrolysis products.

The electrolysis cell 11 is also executed in a three-chamber construction in which the carbon dioxide CO₂ is introduced into the cathode space 14 via the cathode 15 executed as a gas diffusion electrode. Gas diffusion electrodes enable mutual contacting of a solid catalyst, a liquid electrolyte and a gaseous electrolysis reactant. For this purpose, for example, the catalyst may be executed in porous form and assume the electrode function, or a porous electrode assumes the catalyst function. The pore system of the electrode is configured here such that the liquid phase and the gaseous phase can penetrate equally into the pore system and may be present simultaneously therein, i.e. at the electrically accessible surface thereof. One example of a gas diffusion electrode is an oxygen-depolarized electrode.

For configuration as a gas diffusion electrode, the cathode 15 in this example comprises a metal mesh to which a mixture of PTFE, activated carbon and a catalyst has been applied. For introduction of the carbon dioxide CO2 into the catholyte circuit, the electrolysis cell 11 comprises a carbon dioxide inlet 24 into the gas space 16. In the gas space 16, the carbon dioxide reaches the cathode 15, where it can penetrate into the porous structure of the cathode 15 and hence be reacted.

In some embodiments, the arrangement 10 comprises an electrolyte circuit 20, by means of which the anode space 12 and the cathode space 14 are supplied with a liquid electrolyte, for example K2SO4, KHCO3, KOH, Cs2SO4, and the electrolyte is recycled into a reservoir 19. The electrolyte is circulated in the electrolyte circuit 20 by means of a pump 18.

In the present example, the gas space 16 comprises an electrolyte outlet 25 disposed in the base region. The electrolyte outlet 25 leads through a pressure-controlled proportional valve 32 to the reservoir 19. In some embodiments, there is a first pressure sensor 31 that measures the pressure in the gas space 16, and a second pressure sensor 30 for measurement of the pressure in the cathode space 14.

The control device 23 receives the measurement signals from the pressure sensors 30, 31 and ascertains the pressure differential between the cathode space 14 and the gas space 16. If the pressure differential exceeds a definable threshold, the valve 32 is opened, in order that accumulated electrolyte can run out of the gas space 16. The electrolyte is guided back into the reservoir 19. If the pressure differential goes below the threshold value or a second threshold value, the valve is closed.

When starting up the electrolysis, in spite of a positive pressure on the gas side, i.e. in the gas space 16, the electrical voltage applied to the cathode 15 results in “pumping” of electrolyte out of the catholyte space 14 through the gas diffusion electrode, i.e. the cathode 15, in the direction of gas space 16. Droplets form on the side of the gas space 16 at the surface of the cathode 15, which coalesce and collect in the form of a film in the lower region of the cathode 15.

As a result, the accumulating electrolyte causes a pressure rise in the gas space 16, and then, after a short time (about 30 min), the voltage-electrolyte pumping effect through the cathode 15 ceases. No further electrolyte is supplied, the gas space 16 dries out, and the co-transported salt crystallizes out and hence blocks the pores of the cathode 15.

However, by virtue of the operation of the valve 32, the electrolysis is operated in a constant manner within a particular pressure differential range between gas space 16 and the electrolyte. As a result, the “electrolyte pump” through the cathode 15 is maintained and salting-up of the gas diffusion electrode is prevented. At the same time, it is ensured that the electrolyte in the gas space 16 can also be discharged again. 

What is claimed is:
 1. A system for carbon dioxide electrolysis, the system comprising: an electrolysis cell having an anode and a cathode both connected to a voltage supply, wherein the cathode comprises a gas diffusion electrode adjoined on a first side by a gas space and on a second side by a cathode space; an electrolyte circuit adjoining the electrolysis cell; a gas supply for supplying carbon dioxide-containing gas to the gas space, the gas space including an electrolyte outlet; and a shutoff device for the electrolyte outlet, wherein the shutoff device opens when a pressure differential between the gas space and the cathode space exceeds a threshold value.
 2. The system as claimed in claim 1, wherein the shutoff device comprises a pressure relief valve.
 3. The system as claimed in claim 1, wherein the electrolyte outlet is disposed at a end of the gas space.
 4. The system as claimed in claim 1, further comprising a pressure sensor measuring a pressure in the gas space.
 5. The system as claimed in claim 1, further comprising a pressure sensor measuring a pressure in the cathode space.
 6. The system as claimed in claim 1, further comprising a pressure sensor measuring the pressure differential between the gas space and the cathode space.
 7. The system as claimed in claim 1, wherein the electrolyte outlet connects to the electrolyte circuit.
 8. The system as claimed in claim 1, further comprising a control unit to control the shutoff device as a function of the pressure differential.
 9. A method for carbon dioxide electrolysis with an electrolysis cell having an anode and a cathode, both connected to a voltage supply, wherein the cathode comprises a gas diffusion electrode adjoined on a first side by a gas space and on a second side by a cathode space, the method comprising: supplying carbon dioxide-containing gas into the gas space; performing electrolysis in the gas space; measuring a pressure differential between the gas space and the cathode space; and opening a shutoff device in communication with the gas space if the pressure differential exceeds a threshold value.
 10. The method as claimed in claim 9, further comprising operating the shutoff device to maintain the pressure differential within a predefined interval. 